After the completion of a primary blasting program within an underground excavation, there remain large boulders, rocks and pieces of ore. These materials may be found individually scattered on the floor; clumped together; or hung up in boxholes, chutes and ore passes. The size of these blast products must be sufficiently reduced before they can be subsequently handled.
Accordingly, secondary blasting is undertaken to break up the residual oversize boulders. There are generally three methods used in secondary blasting:
1) Blockholing--one or more holes are drilled into a boulder and filled with an explosive. The explosive is detonated thereby fracturing the rock. This method, although economical in the use of explosives, involves considerable expense in preparation and labor. It often causes subsequent damage due to flyrock ruining surrounding air and water lines, ventilation tubing, support screens, etc. Additional dangers may be experienced by drilling into missed holes or caused by moving muck. It is also quite dangerous to drill upwardly into a hanging boulder inasmuch as the roof or sidewalls may be unstable. PA1 2) Sandblasting--placing an explosive charge on a boulder under a blanket of sand (the process may be also carried out without sand). This usually requires a larger quantity of explosives than blockholing with the attendant increased damages to the surrounding area. Reblasting is frequently required because the charge is inefficiently shaped and improperly placed. Time is further wasted since the charge must be made up at the site. PA1 3) Concussion, boxhole, chute or ore pass blasting--these require access to notoriously unstable rock formations located in the chutes, boxholes and ore passes present in underground excavations. It is unsafe to work under hung up chutes and passes so the usual procedure is to hang a primed charge at the end of a pole and insert the charge and pole into the chute or pass. This is inefficient since the hanging charge may have little or no contact with the trapped rock.
The latter two methods transfer high energy to the stope walls causing potential rock falls and loose ground conditions. As with sandblasting, concussion blasting may also damage mine services--water, air and electrical lines, ventilation tubing, etc.
Concussion and sandblasting attempt to break rock by using the explosive as a "giant hammer" to explode against or adjacent to the rock and fracture it.
An example of a secondary blasting charge is disclosed in U.S. Pat. No. 2,247,169. A flexible capsule having a flange is filled with an explosive. A circumferential rabbet holds the explosive in place. An adhesive is placed on the flange and the entire unit is placed against the rock with the flange adhesive holding the unit to the rock.
Unfortunately, due an air gap formed between the explosive and the rock, a decided loss of shock energy may be experienced thereby reducing the effectiveness of the charge in spite of the attached flange.
Essentially, there is a need for a secondary explosive that is relatively fast to set up, less labor intensive and costly than current techniques, less wasteful of explosive energies normally dissipated and possibly directed against mine services, and perhaps most importantly safe to use.